This invention relates to a method of etching a semiconductor substrate to a desired depth from the principal surface in a selected area or selected areas by immersing the semiconductor substrate with a patterned mask in an etching liquid.
In producing semiconductor devices it is often necessary to etch at least one selected area of a semiconductor substrate to a desired depth from the principal surface by immersing the semiconductor substrate in an etching liquid after providing the substrate with a mask having a window for each area to be etched.
For example, a semiconductor substrate using a single crystal of silicon so as to make its (100) crystal plane the principal surface of the substrate is etched in 100% hydrated hydrazine. In advance of immersing into the etching liquid, a silicon oxide film is formed on each side of the semiconductor substrate by a heat treatment in an oxidizing atmosphere, and by a photo-etching process the oxide film on the top surface is removed in the area(s) where the substrate is to be etched.
In this method, etching is accomplished by using oxidation-reduction reactions between the etching liquid and the exposed surface of the semiconductor substrate. That is, etching proceeds as silicon in the exposed area of the substrate is oxidized by hydrazine and dissolves in the hydrated hydrazine as silicon ions. This is the phenomenon called anodic dissolution. However, the etching process is complicated by a cathodic reduction reaction which accompanies the anodic oxidation dissolution reaction. Since etching proceeds under a mixed potential condition imposed by the oxidation and reduction reactions, there is a strong tendency toward formation of local cells within the area to be etched, and the local cells become a serious cause of local nonuniformity of reactions. More particularly, while etching by the anodic dissolution is proceeding in some regions of the etching area of the semiconductor substrate, a reduction reaction preferentially takes place in other regions of the same area to cause precipitation of a portion of silicon dissolved in the etching liquid. Consequently, a number of tiny pyramid-shaped projections, each of which has surfaces of (111) crystal planes, appear in the new surface provided by the etching process, so that the flatness of the new surface is significantly marred.
A conventional technique to cope with the above described problem is addition of a suitable quantity of reaction buffering additive such as water, surfactant or alcohol to the 100% hydrated hydrazine used as etching liquid. However, the addition of such an additive lowers the etch rate with a resultant lowering of productivity of the semiconductor device manufacturing process. Another known measure is optimizing the environmental conditions of etching such as the rate of stirring of the etching liquid and the temperature of the etching liquid. However, even when such conditions are optimized it is inevitable that the above described pyramid-shaped projections are formed as the concentration of dissolved matter in the etching liquid increases as progresses.